How to add a hdr world background to its own layer?
The way I did this was to do a dummy run render of the camera's motion within the environment on an empty scene layer, with the environment set to be "visible" to camera, and saved this as a file of PNG's.  Then set up your scene proper, set the film to "transparent", check "use environment" on your render layers, and set the environment itself not to be visible to the camera.  Finally, in the compositor, bring the environment back in as an image (set to image sequence), and alpha over the rest of your scene.